A radio frequency coil arrangement as well as a probe head of the type specified above are disclosed in an article of Stringer, John A. et al. “Reduction of RF-induced sample heating with a scroll coil resonator structure for solid state NMR probes”, Journal of Magnetic Resonance, 173, (2005), pp. 40-48, as well as in an article of Grant, S. C. “Analysis of Multilayer Radio Frequency Microcoils for Nuclear Magnetic Resonance Spectroscopy”, IEEE Transactions on Magnetics, 37, (2001), pp. 2989-2998.
Japanese patent application publication JP 01-046 637 A discloses a spiral resonator. This spiral resonator, in a plane, developed view, consists of a narrow, rectangular metal sheet which is then wound such that a circular loop is generated in which the ends of the wound metal sheet slightly overlap and are arranged at a small distance from one another. The resonator is intended to be used for electron resonance (ESR) and nuclear resonance (NMR) measurements, in particular for measurements on samples having high dielectric losses.
Coil arrangements and probe heads of the type of interest in the context of the present invention are, preferably, used for nuclear magnetic resonance (NMR) measurements on small samples having losses. However, this does not exclude the application of the invention to other methods, in particular electron spin resonance (ESR).
Insofar, the small volume may result, on the one hand, from the fact that there are just only little amounts of sample substance available. On the other hand, the sensitivity of magnetic measurements increases, as is well known, when the measuring frequency is increased, or the constant, homogeneous magnetic field strength, resp., in which the samples are located. The higher the measuring frequency or the smaller the wavelength, resp., the smaller are the dimensions of the coil or resonator arrangements receiving the sample. The term “small sample” is to be understood to mean sample volumes of the order of magnitude of 50 μl. Such samples, typically, have dielectric losses, when it comes to liquid, semi-liquid or salty solid state samples. The lossy samples effect a decrease of the quality factor as well as a detuning of the frequency in the radio frequency coil arrangement used, all resulting in reduced sensitivity.
The conductivity of the sample substance in such cases results in a coupling with the electrical radio frequency field. When conceiving coil arrangements and probe heads for magnetic resonance measurements, one has, therefore, the desire to configure the spatial distribution of the electrical field on the one hand and the spatial location of the sample substance on the other hand, such that the smallest possible zones of overlap exist.
In conventional solenoid coils a wire-shaped conductor is helically wound about a cylindrical volume into which a sample container may be inserted. If a radio frequency signal is fed thereto, the radio frequency magnetic field permeates the volume and, hence, the sample, in an axial direction. However, when doing so, a non-negligible electrical stray field occurs which, in connection with samples of the type mentioned above, results is substantial dielectric losses with the consequences lined out above.
It has turned out that this problem becomes the more severe, the smaller the coil and the sample volume, resp., are made. When the sample volumes are very small, i.e. in the range of several 10 μl, then sensitivity may drop to unacceptable values.
From the articles of Stringer and of Grant, cited at the outset, wound coils, the so-called “scroll coils”, have become known. These coils are made by spirally winding a small rectangle shaped and electrically conductive stripe about an axis. The scroll coil terminals or connectors are then located at the inner and at the outer narrower sides of the wound stripe. This coil configuration is characterized by a smaller electrical stray field in the area close to the coil axis. It has, therefore, turned out to be advantageous for small sample volumes.
The prior art scroll coils, however, have the disadvantage that due to the extremely confined space it is difficult to make the terminal at the inner narrow side of the wound stripe, in particular for the very small coils as are of interest in the present context. The cross-wise outward connection of the terminal from the area close to the axis, further, results in an asymmetrical arrangement. When the latter is driven asymmetrically, the inner terminal is electrically “cold” (low radio frequency voltage) and the outer terminal is electrically “hot” (high radio frequency voltage).